1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member and a producing method therefor.
2. Related Background Art
As one of members employed in an electrophotographic photosensitive member, a non-single crystalline deposition film containing silicon atoms as a principal component, for example a deposition film of an amorphous silicon (hereinafter represented as a-Si) compensated with hydrogen and/or halogen, is proposed as a photosensitive member having high performance, high durability and pollution-free properties, and some of such members are already in practical use.
Such a-Si electrophotographic photosensitive members are proposed having various layered structures for meeting requirements for various properties, and in particular a surface layer is recognized as an important layer for realizing various properties of the electrophotographic photosensitive members, such as abrasion resistance, charge holding properties, environmental resistance, light-transmitting properties and the like.
As a shorter wavelength is contemplated for image exposure for achieving a higher definition in a copying apparatus, the surface layer is particularly required to have characteristics of a wide band gap which can transmit short-wavelength light with less absorption.
As materials capable of satisfying such characteristics, for example, a metal fluoride and a silicon nitride may be cited.
As an electrophotographic photosensitive member utilizing such a material, a structure is disclosed utilizing magnesium fluoride in the surface layer (for example cf. Japanese Patent Application Laid-open No. 2003-29437).
Japanese Patent Application Laid-open No. 2003-29437 discloses that magnesium fluoride employed as the surface layer provides excellent properties such as inhibition of image blur or image deletion, and potential variation due to abrasion.
Thus, not only magnesium fluoride but also metal fluorides in general are expected to provide properties preferred as the material of the surface layer of the electrophotographic photosensitive member owing to hardness, light-transmitting properties and low surface energy.
On the other hand, from the standpoint of a process for forming a deposition film, a plasma CVD process is preferable for forming a photoconductive layer or the like because a relatively large film thickness is required and also because a raw material gas is easily available, but a raw material for the surface layer, usable in the plasma CVD process, is not necessarily easily available. For example, in a metal material usable as a raw material for a metal fluoride, raw material atoms are difficult to supply from a gaseous substance, so that a PVD process, typified by sputtering, is suitable for forming the surface layer.
Therefore, the afore-mentioned patent literature discloses an example of forming an electrophotographic photosensitive member constituted of a lower blocking layer, a photoconductive layer, a buffer layer and a surface layer, by utilizing a plasma CVD process for the lower blocking layer, the photoconductive layer and the buffer layer and a sputtering for the surface layer.
It is thus rendered possible, by employing an optimum forming process for each layer, to obtain a practical electrophotographic photosensitive member capable of improving electrical and optical characteristics, resistance to environmental conditions and durability, and further to achieve a higher definition of images.